For example, Japanese Unexamined Patent Publication No. 2003-244792 (Patent Document 1) has disclosed a transparent piezoelectric film speaker that can be disposed on a display of a portable telephone. This Patent Document 1 describes an example in which a PVDF (polyvinylidene fluoride) film is used as the piezoelectric film installed in the piezoelectric film speaker. Moreover, Patent Document 1 also describes that the use of a polymer piezoelectric film is preferable since no such brittleness as caused in ceramic materials is exerted. Moreover, Patent Document 1 exemplifies ITO (Indium Tin Oxide) as a material for a transparent electrode to be formed on the piezoelectric film, and also exemplifies a method such as a vacuum vapor deposition method and a sputtering method, as the method used for the formation thereof.
The above-mentioned ITO electrode can be advantageously used in LCD's (liquid crystal panels) and PDP's (plasma displays); however, in recent years, together with rapid spreads of FPD's (flat panel displays), there have been increasing demands for ITO electrodes for use in LCD's and PDP's. In this case, however, since indium contained in ITO is a rare metal, its starvation has been worried about, and its price has been soaring.
Moreover, upon sputtering ITO, a temperature close to about 200° C. is required. For this reason, it becomes difficult to apply the ITO sputtering upon forming an electrode on a sheet made from an organic polymer.
On the other hand, a conductive polymer containing thiophene in its molecule structure becomes substantially transparent when formed into a thin film, and the application of this material for electrodes has been described in, for example, Japanese Unexamined Patent Publication No. 2000-2899 (Patent Document 2), Japanese Unexamined Patent Publication No. 2000-10125 (Patent Document 3) and Japanese Patent Application National Publication (Laid-Open) No. 2001-504976 (Patent Document 4).
In the case of the conductive polymer containing thiophene in its molecule structure, no such problem of resource starvation as in the case of the above-mentioned ITO is raised. Moreover, examples of the conductive polymer containing thiophene in its molecule structure include polythiophene and polyethylene dioxythiophene, and since these materials can be handled in a state of an aqueous solution, by thinly applying this aqueous solution by the use of a method such as a spin-coating method and an ink-jet printing method, and by simply drying the solution, it is possible to form a film having a conductive characteristic. Therefore, this method is suitably applied to an electrode formation on a sheet made from an organic polymer.
However, as described in Patent Documents 2 to 4, the conductive polymer containing thiophene in its molecule structure is slightly colored. In general, the solution of the conductive polymer containing thiophene in its molecule structure has a dark blue color, and although it becomes transparent after the film formation, it has a slightly bluish color. For this reason, when used as an electrode material for a sheet-type vibrator, such as a transparent piezoelectric film speaker to be disposed on a display, it still has some room for improvement.
Patent Document 1: Japanese Unexamined Patent Publication No. 2003-244792
Patent Document 2: Japanese Unexamined Patent Publication No. 2000-2899
Patent Document 3: Japanese Unexamined Patent Publication No. 2000-10125
Patent Document 4: Japanese Patent Application National Publication (Laid-Open) No. 2001-504976